58 CARNEGIK INSTITUTION 



erable height above the ground, at a site like Mount Wilson, where 

 the atmospheric conditions during the day are exceptionally fine. 

 Under such circumstances a sharp and well defined solar image, 

 from two to three feet in diameter, could be obtained, and numerous 

 researches now entirely out of reach could be undertaken with every 

 reason to hope for success. 



Dr. Elihu Thomson has suggested that any distortion of the mir- 

 rors by the sun's heat can be obviated by making them of fused 

 quartz, since the coefl&cient of expansion of this substance is almost 

 inappreciable. Since Dr. Thomson has already succeeded in mak- 

 ing small mirrors in this way, we include an item to cover the ex- 

 pense of the necessary experiments, which Dr. Thomson has very 

 kindly volunteered to superintend. 



(2) In work on the sun, as already remarked, the most important 

 requirement is a large solar image produced by a telescope of great 

 focal length, but in order to trace out the successive stages in the 

 development of stars like the sun, a telescope of a very different 

 type is required. The remarkable opportunities for advance in 

 astronomy which exist at the present time through the possibility 

 of building a large reflecting telescope were outlined in the Year 

 Book of the Carnegie Institution for 1902 (p. 141). The optical 

 parts of the Snow horizontal telescope consist exclusively of mir- 

 rors, and it thus preserves the peculiar advantages of the reflecting 

 telescope ; but for the photography of faint nebulae, and for many 

 other similar researches of fundamental importance in the study of 

 the sun's origin and development, this type of telescope is not well 

 adapted. What is needed is a mirror of the largest possible diam- 

 eter and of short focal length, provided with a heavy and well con- 

 structed equatorial mounting. 



To give an idea of the immense advantages of an instrument of 

 this kind it may be recalled that with a two foot reflector an expos- 

 ure of 40 minutes sufl&ces to photograph stars that are invisible in 

 the largest refracting telescopes. With longer exposures millions 

 of stars can be photographed with such a reflector, of whose exist- 

 ence the largest refractors could never give any indication. A 5-foot 

 reflecting telescope would collect six times as much light as a 2-foot, 

 and nearly three times as much as the largest reflector now in use, 

 and would open up certain fields of investigation now entirely closed. 

 It would furnish means of photographing the nebulae which would 

 probably be superior to those offered by any existing telescope. It 

 would also permit the heat radiation of some of the brighter stars 



